1. Field of the Invention
The present invention is directed to improving the Bayer process; and, more particularly, to reducing the foam formed in the liquor of the Bayer process.
2. Description of Prior Art
In the Bayer process for the production of alumina, bauxite ore is pulverized, slurried in water, and then digested with caustic at elevated temperatures and pressures. The caustic solution dissolves oxides of aluminum, forming an aqueous sodium aluminate solution. The caustic-insoluble constituents of bauxite ore (referred to as "red mud") are then separated from the aqueous phase containing the dissolved sodium aluminate. Solid alumina trihydrate is precipitated out of the solution and collected as product.
In more detail, the pulverized bauxite ore is fed to a slurry mixer where a water slurry is prepared. The slurry makeup water is typically spent liquor (described below) and added caustic. This bauxite ore slurry is then diluted and passed through a digester or a series of digesters where, under high pressure and temperature, about 98% of the total available alumina is released from the ore as caustic-soluble sodium aluminate. After digestion, the slurry then passes through several flash tanks wherein the pressure of the digested slurry is reduced from several atmospheres to one atmosphere and the temperature of the slurry is reduced from about 400.degree. F. to about 220.degree. F.
The aluminate liquor leaving the flashing operation contains from about 1 to about 20 weight percent solids, which solids consist of the insoluble residue that remains after, or is precipitated during, digestion. The coarser solid particles may be removed from the aluminate liquor with a "sand trap" cyclone. The finer solid particles are generally separated from the liquor first by settling and then by filtration, if necessary. Any Bayer process slurry taken from the digesters through any subsequent dilution of the slurry, including the flash tanks, but before the primary settler, is referred hereinafter as the primary settler feed. The slurry of aluminate liquor leaving the flash tanks is diluted by a stream of recycled wash overflow liquor.
Normally, the primary settler feed is thereafter fed to the center well of the primary settler, where it is treated with a flocculant. As the mud settles, clarified sodium aluminate solution, referred to as "green" or "pregnant" liquor, overflows a well at the top of the primary settler and is collected. This overflow from the primary settling tank is passed to the subsequent process steps. The treatment of the liquor collected after the primary settlement to remove any residual suspended solids before alumina trihydrate is recovered is referred to as a secondary clarification stage.
The clarified sodium aluminate liquor is seeded with alumina trihydrate crystals to induce precipitation of alumina in the form of alumina trihydrate, Al(OH).sub.3. The alumina trihydrate particles or crystals are then separated from the concentrated caustic liquor, and the remaining liquid phase, the spent liquor, is returned to the initial digestion step and employed as a digestant after reconstitution with caustic.
Because of the organic content of Bayer liquor, it has a natural tendency to foam. The foaming of the liquor is aggravated by mechanical agitation, by airsparging, and by transfer of the liquor from one vessel to the next. Foaming generally occurs after separation of the red mud, and before and during the precipitation of alumina trihydrate. Foam can also develop in transfer points. Foam can occur at any point after the digestion step where the pressure of the digested slurry is reduced to 1 atmosphere. Foaming is especially a problem after separation of the red mud.
The foam poses safety hazards in that the overflow of foam on vessel surfaces is a hazard to process workers since the foam is extremely caustic. The workers would suffer chemical burns upon contact with the foam. It is critical to eliminate or reduce the foam because employee safety is very important to both the employee and the process operator.
The foam also complicates the heat control of the process. Because a vessel surface covered with foam serves as an insulator which retards heat loss, thermal control of the process is difficult. This is especially important because processors strive to reduce liquor temperature during precipitation in order to maximize yield of the product alumina trihydrate.
Vessels filled with large amounts of foam cannot be filled with the maximum quantity of liquor. It is important to fill the vessel completely with liquor in order to maximize product yield and process efficiency. In light of the above safety, engineering and economic problems caused by Bayer process foam many have attempted to ameliorate the problem of foaming.
A variety of treatment types have been employed in the past, including alcohols, glycols, silicon compounds, hydrophobic silica, wax emulsions and fatty acid based treatments. Many of the above chemical treatments do not work in all Bayer process liquors. Of the treatments that do work, many are not persistent antifoam/defoamers, in that the activity diminishes as the treatment chemical moves through consecutive precipitation vessels.
The cost of the above chemical treatments is also high. In a competitive economic climate, a defoamer/antifoam composition having a reduced cost would provide a great economic benefit.
Several of the prior art antifoam/defoamer treatments are malodorous, volatile or hazardous to workers in Bayer process facilities. Operators are actively seeking antifoam/defoaming treatments which are more worker-friendly and environmentally benign.
Several of the antifoam/defoamer compositions listed above, such as the fatty acid based materials, adversely affect crystal size distribution. Also, some antifoam/defoamer compositions such as silica or silicone introduce an undesirable impurity into the Bayer liquor which can reduce the purity, quality and salability of the product alumina trihydrate. This interferes with the control of the process. Accordingly, process operators would like to replace these antifoam/defoaming treatments with a different treatment which does not negatively impact the overall control of the process.
It is well known that polypropylene glycols with molecular weights in excess of about 1,000 daltons are good antifoams because of their limited water-solubility. For example, polypropylene glycol of molecular weight 4,000 is known to have commerical utility as an antifoam in Bayer liquors. One problem with high molecular weight polypropylene glycol preparation currently available is that they are not water-soluble. Water is advantageous as a carrier because it is economical and does not introduce additional organic impurities into the Bayer liquor.
In light of the enormous difficulties posed by the foaming problem in Bayer process liquors and the inadequate antifoam/defoaming tretments currently available, it would be advantageous to provide an antifoam/defoamer composition which prevents or ameliorates Bayer process foam without the problems caused by currently available antifoam/defoamer treatments.